1. Field of the Invention
The present invention relates to a magnet assembly for generating the basic magnetic field in a magnetic resonance imaging (MRI) device.
2. Description of the Prior Art
A magnet is a component of a magnetic field generation unit of an MRI device. The magnet in such an assembly has a patient tube in which a patient lies during his/her examination. The magnetic field generation unit generates a substantially homogenous and temporally constant (main) magnetic field (also called a basic magnetic field or B.sub.0 field) in an examination volume (imaging volume) situated inside the patient tube. Because of the necessary homogeneity of the examination volume, this is also called a homogeneity volume.
The minimum required extent of the generally spherical homogeneity volume is determined by the fact that signal pickups must be possible at spatially varied positions of the patient. Thus, for example, head surveys are performed centered with respect to the magnetic longitudinal axis, while heart imaging occurs outside the magnetic longitudinal axis. The required minimum extent of the homogeneity volume thus must be relatively large, although the examined organs or body regions themselves would require only a small homogeneity volume.
The extent of the homogeneity volume is essentially determined by the length of the magnet. The size (length and diameter) of the magnet determines the cost of the magnet, with a larger size naturally resulting in a correspondingly higher cost due to the basis of the required basic homogeneity. Furthermore, in long patient tubes, which are at least as long as the patient himself or herself, the patient may experience anxiety during the examination.